(a) Field of the Invention
The present invention concerns the processes for supplying gas to a very sensitive analyzer, for detecting traces of residual gases in a pure gas (herein after called "traces"), of the type comprising the sequential steps of supplying to a section of an input line to the analyzer, a given flow:
a) of a gas to be analyzed, via a first line, PA1 b) of a pure gas prepared by purifying the gas to be analyzed, via a second line, PA1 c) at least one calibration gas, which is diluted with the pure gas, and which initially contains larger traces of residual gases than the gas to be analyzed, via a third line, PA1 the downstream section of the three lines all meeting the section of the input line. PA1 a source of gas to be analyzed; PA1 a source of calibration gas; PA1 a first line, including a first flow regulator means, directly connecting the source of gas to be analyzed to a section of the input line of the analyzer; PA1 a second line, connected upstream to the source of gas to be analyzed and including in series a gas purifier and a second flow regulator means; and PA1 at least a third line, including a third flow regulator means, connected to a source of calibration gas initially containing a larger quantity of traces than the gas to be analyzed, and downstream, to a downstream part of the second line, PA1 characterized in that each line includes, in an upstream part, a calibrated restriction defining a sonic throat and, in an intermediate part thereof, a discharge by-pass provided with a discharge flow regulator.
(b) Description of Prior Art
The utilization of very sensitive analyzers for detecting traces of impurities, such as of the ultrasensitive hygrometer or Fourier transformed infrared spectroscope (FTIR) type, which are capable of detecting and analyzing traces of gases at levels lower than ppb and which are used for example for very high purity gases (nitrogen, argon) in the very highly integrated semi-conductor industry (VLSI), requires being able to sequentially supply to the analyzer, at a given flow, the gas to be analyzed, a so-called "zero" gas, obtained by purification of the gas to be analyzed and qualifying the zero of the analyzer, and a calibration gas, obtained by a high dilution of a gas initially containing a given and important quantity of traces, so as to calibrate the reading range of the analyzer.
The known processes and devices utilize three lines each provided with a flow regulator and a stop valve to isolate the lines which are of no interest in one of the steps for supplying gas to the analyzer. These processes have two major disadvantages because the flow regulators constitute polluting components which are susceptible to add uncontrolled traces or impurities to the various lines of gas, and during the periods where the lines are isolated, the gas present therein is susceptible to be rapidly polluted by mere absorption or desorption through the walls of the tubes.
To substantially reduce the above disadvantages, a process and a device have been proposed where each line is provided with a loss of static charge, which is less polluting than a flow regulator, and where, in the downstream section of the line, a three-way valve enables the isolation and flushing of the upstream section of the line. This approach enables reducing the tube lengths which are isolated and have not been flushed but does not completely remove these lengths. On the other hand, the feeding pressure into the first and second lines may vary within a wide range, so that the proposed process and device present the risks of an important time lag not permitting assuring the supply of a given flow to the analyzer, and consequently, possibly falsifying the precision of the measurements.